The present invention relates to a pulse oximeter which can measure an oxygen saturation of arterial blood continuously and non-invasively by utilization of variations in the volume of arterial blood by pulsation.
A pulse oximeter has hitherto been widely known as an apparatus which measures an oxygen saturation of blood; more specifically, a concentration ratio of oxyhemoglobin to a sum of oxyhemoglobin and deoxyhemoglobin. Japanese Patent Publication No. 5-88609B discloses an apparatus for determining concentrations of constituents of blood with high precision without being affected by pure tissue whose thickness varies due to pulsation of blood.
More specifically, this publication discloses an apparatus for determining concentrations of constituents of blood comprising: a light emitter which irradiates a living tissue with light beams of N different wavelengths; a light receiver which receives light originated from the light emitter and then reflected from or transmitted through a living tissue; a first circuit which detects an attenuation change at the living tissue based on output signals from the light receiver for each of the N wavelengths; a second circuit which calculates N−1 attenuation change ratios between different wavelengths, in accordance with detection signals pertaining to the N different wavelengths output from the first circuit; and a third circuit for calculating which calculates relative concentrations of N−1 constituents of blood, through use of arithmetic equations determined by solving simultaneous equations with N−1 unknowns in connection with concentrations of constituents of blood, on the basis of values of attenuation change ratios output from the second circuit. This calculation is based on the assumption that attenuation changes in the living tissue stem from changes in the thickness of blood and thickness of pure tissue which does not include blood.
In the above apparatus, the second circuit enables calculation of a ratio between N−1 pulsation changes in different wavelengths when the second circuit receives signals output from the first circuit. The third circuit performs computation by substituting actually-measured values of ratios of pulsation changes and respective coefficient values into an equation for determining relative concentrations of N−1 constituents of blood. The equation is obtained by solving simultaneous equations pertaining to N−1 pulsation change ratios that also take into consideration the influence of pulsation of pure tissue. Accordingly, concentrations (relative concentrations) of N−1 constituents of blood can be measured with high precision and without being influenced by pulsation of pure tissue.
Meanwhile, when body motion occurs during measurement of concentrations of constituents of blood by a pulse oximeter, artifacts are superposed on the transmitted light. When the artifacts stemming from such body motion arm large, difficulty is encountered in removing the artifacts by correcting pulse waveforms, or the like. Japanese Patent Publication No. 11-216133A discloses a pulse oximeter capable of performing high-precision measurement without being influenced by the artifacts stemming from body motion even when the artifacts are large.
More specifically, the pulse oximeter comprises: a light emitter which irradiates a living tissue with a plurality of light beams having different wavelengths; a photoelectric converter which converts a light beam transmitted through the living tissue into an electric signal for each of the wavelengths; a first detector which detects an attenuation change at the living tissue based on fluctuations of the signals output from the photoelectric converter for each of the wavelengths; a variable filter which receives the attenuation change for each of the wavelengths and allows to pass through a component having a prescribed frequency band; a band prescriber which prescribes the frequency band for the variable filter; and a second detector which determines an oxygen saturation in accordance with an output from the variable filter.
The apparatus disclosed in Japanese Patent Publication No. 5-88609B teaches that a plurality of light beams having different wavelengths to precisely measure concentrations of constituents of blood without being influenced by pure tissue whose thickness varies due to pulsation of blood. The influence of a term of the pure tissue can be eliminated by use of light beams of three different wavelengths during the measurement process, thereby enabling measurement of a relative concentration between oxyhemoglobin and deoxyhemoglobin, which are two constituents of blood.
The influence of a term of the pure tissue can be eliminated by use of light beams of four different wavelengths during the measurement process, as in the case of the above, thereby enabling precise measurement of relative concentrations between oxyhemoglobin, deoxyhemoglobin and another pigment, which are three constituents of blood. Furthermore, use of light beams of five different wavelengths enables measurement of relative concentrations between four constituents of blood including carboxyhemoglobin in addition to the above three constituents of blood measured by use of the four different light wavelengths.
However, this publication is silent about the influence of the artifacts stemming from body motion. Therefore, means for eliminating the influence of the artifacts is not disclosed at all.
Meanwhile, Japanese Patent Publication No. 11-216133A teaches means for eliminating the influence of the artifacts stemming from body motion. However, in a pulse oximeter disclosed in this publication, changes in the thickness of arterial blood and changes in the thickness of pure tissue, which are assumed to be causes of the aromas stemming from body motion, are regarded as negligible factors. Focus is paid to changes in the thickness of venous blood as the paramount cause, and measures are taken against the changes in the thickness of venous blood. In summary, this publication teaches a method and an apparatus for performing: a two-wavelength measurement which disregards changes in tissue thickness; and a three-wavelength measurement which takes into consideration changes in tissue thickness.
Accordingly, the most serious problem faced by a currently available pulse oximeter is the artifacts stemming from body motion. A primary method adopted as a countermeasure for avoiding the influence of the artifacts stemming from body motion is a statistical method. Specifically, reference is made to antecedent data and subsequent data in order to obtain a reliable measurement value at a certain point in time. Therefore, such a measurement entails a time delay and value smoothing. This is against the primary object of a pulse oximeter to detect anomalies in a patient at an early stage. That is, a function for eliminating the influence of artifacts is insufficient or involves inconveniences.